Powder metallurgical techniques enable producing parts with complicated shapes in shapes that are extremely close to product shapes (so-called near net shapes) with high dimensional accuracy, and consequently significantly reducing machining costs. For this reason, powder metallurgical products are used for various machines and parts in many fields.
In recent years, there is a strong demand for powder metallurgical products to have improved toughness in terms of improving the strength for miniaturizing parts and reducing the weight thereof and safety. In particular, for powder metallurgical products (iron-based sintered bodies) which are very often used for gears and the like, in addition to higher strength and higher toughness, there is also a strong demand for higher hardness in terms of wear resistance. In order to meet the above-mentioned demands, iron-based sintered bodies of which components, structures, density and the like are controlled suitably are required to be developed, since the strength and toughness of an iron-based sintered body varies widely depending on those properties.
Typically, a green compact before being subjected to sintering is produced by mixing iron-based powder with alloying powders such as copper powder and graphite powder and a lubricant such as stearic acid or lithium stearate to obtain mixed powder; filling a mold with the mixed powder; and compacting the powder.
The density of a green compact obtained through a typical powder metallurgical process is usually around 6.6 Mg/m3 to 7.1 Mg/m3. The green compact is then sintered to form a sintered body which in turn is further subjected to optional sizing or cutting work, thereby obtaining a powder metallurgical product. Further, when even higher strength is required, carburizing heat treatment or bright heat treatment may be performed after sintering.
Based on the components, iron-based powders used here are categorized into iron powder (e.g. iron-based powder and the like) and alloy steel powder. Further, when categorized by production method, iron-based powders are categorized into atomized iron powder and reduced iron powder. Within these categories specified by production methods, the term “iron powder” is used with a broad meaning encompassing alloy steel powder as well as iron-based powder.
In terms of obtaining a sintered body with high strength and high toughness, it is advantageous that iron-based powder being a main component in particular allows alloying of the powder to be promoted and high compressibility of the powder to be maintained.
First, known iron-based powders obtained by alloying include:
(1) mixed powder obtained by adding alloying element powders to iron-based powder,
(2) pre-alloyed steel powder obtained by completely alloying alloying elements,
(3) partially diffusion alloyed steel powder (also referred to as composite alloy steel powder) obtained by partially adding alloying element powders in a diffused manner to the surface of particles of iron-based powder or pre-alloyed steel powder.
The mixed powder (1) mentioned above advantageously has high compressibility equivalent to that of pure iron powder. However, in sintering, the alloying elements are not sufficiently diffused in Fe and form a non-uniform microstructure, which would result in poor strength of the resulting sintered body. Further, since Mn, Cr, V, Si, and the like are more easily oxidized than Fe, when these elements are used as the alloying elements, they get oxidized in sintering, which would reduce the strength of the resulting sintered body. In order to suppress the oxidation and reduce the amount of oxygen in the sintered body, it is necessary that the atmosphere for sintering, and the CO2 concentration and the dew point in the carburizing atmosphere are strictly controlled in the case of performing carburizing after sintering. Accordingly, the mixed powder (1) mentioned above cannot meet the demands for higher strength in recent years and has become unused.
On the other hand, when the pre-alloyed steel powder obtained by completely alloying the elements of (2) mentioned above is used, the alloying elements can be completely prevented from being segregated, so that the microstructure of the sintered body is made uniform, leading to stable mechanical properties. In addition, also in the case where Mn, Cr, V, Si, and the like are used as the alloying elements, the amount of oxygen in the sintered body can be advantageously reduced by limiting the kind and the amount of the alloying elements. However, when the pre-alloyed steel powder is produced by atomization from molten steel, oxidation in the atomization of the molten steel and solid solution hardening of steel powder due to complete alloying would be caused, which makes it difficult to increase the density of the green compact after compaction (forming by pressing). When the density of the green compact is low, the toughness of the sintered body obtained by sintering the green compact is low. Therefore, also when the pre-alloyed steel powder is used, demands for higher strength and higher toughness cannot be met.
The partially diffusion alloyed steel powder (3) mentioned above is produced by adding alloying elements to iron-based powder or pre-alloyed steel powder, followed by heating under a non-oxidizing or reducing atmosphere, thereby partially diffusion bonding the alloying element powders to the surface of particles of iron-based powder or pre-alloyed steel powder. Accordingly, advantages of the iron-based mixed powder of (1) above and the pre-alloyed steel powder of (2) above can be obtained.
Thus, when the partially diffusion pre-alloyed steel powder is used, oxygen in the sintered body can be reduced and the green compact can have a high compressibility equivalent to the case of using pure iron powder. Therefore, the sintered body has a multi-phase structure consisting of a completely alloyed phase and a partially concentrated phase, increasing the strength of the sintered body.
As basic alloy components used in the partially diffusion alloyed steel powder, Ni and Mo are used heavily.
Ni has the effect of improving the toughness of a sintered body. Adding Ni stabilizes austenite, which allows more austenite to remain as retained austenite without transforming to martensite after quenching. Further, Ni serves to strengthen the matrix of a sintered body by solid solution strengthening.
Meanwhile, Mo has the effect of improving hardenability. Accordingly, Mo suppresses the formation of ferrite during quenching, allowing bainite or martensite to be easily formed, thereby strengthening the matrix of the sintered body. Further, Mo is contained as a solid solution in a matrix to solid solution strengthen the matrix, and forms fine carbides to strengthen the matrix by precipitation.
As an example of the mixed powder for high strength sintered parts using the above-described partially diffusion alloyed steel powder, JP 3663929 B2 (PTL 1) discloses mixed powder for high strength sintered parts obtained by mixing Ni: 1 mass % to 5 mass %, Cu: 0.5 mass % to 4 mass %, and graphite powder: 0.2 mass % to 0.9 mass % to alloy steel powder in which Ni: 0.5 mass % to 4 mass % and Mo: 0.5 mass % to 5 mass % are partially alloyed. The sintered material described in PTL 1 contains 1.5 mass % of Ni at minimum, and substantially contains 3 mass % or more of Ni according to Examples of PTL 1. This means that a large amount of Ni as much as 3 mass % or more is required to obtain a sintered body having a high strength of 800 MPa or more. Further, obtaining a material having a high strength of 1000 MPa or more by subjecting a sintered body to carburizing, quenching, and tempering also requires a large amount of Ni as much as for example 3 mass % or 4 mass %.
However, Ni is an element which is disadvantageous in terms of addressing recent environmental problems and recycling, so its use is desirably avoided as possible. Also in respect of cost, adding several mass % of Ni is significantly disadvantageous. Further, when Ni is used as an alloying element, sintering is required to be performed for a long time in order to sufficiently diffuse Ni in iron powder or steel powder. Moreover, when Ni being an austenite phase stabilizing element is not sufficiently diffused, a high Ni concentration area is stabilized as the austenite phase (hereinafter also referred to as γ phase) and the other area where Ni is hardly contained is stabilized as other phases, resulting in a non-uniform metal structure in the sintered body.
As a Ni-free technique, JP 3651420 B2 (PTL 2) discloses a technique associated with partially diffusion alloyed steel powder of Mo free of Ni. That is, PTL 2 states that optimization of the Mo content results in a sintered body having high ductility and high toughness that can resist repressing after sintering.
Further, regarding a high density sintered body free of Ni, JP H04-285141 A (PTL 3) discloses mixing iron-based powder having a mean particle diameter of 1 μm to 18 μm with copper powder having a mean particle diameter of 1 μm to 18 μm at a weight ratio of 100:(0.2 to 5), and compacting the mixed powder and sintering the green compact. In the technique disclosed in PTL 3, iron-based powder having a mean particle diameter that is extremely smaller than that of typical one is used, so that a sintered body having a density as extremely high as 7.42 g/cm3 or more can be obtained.
WO 2015/045273 A1 (PTL 4) discloses that a sintered body having high strength and high toughness is obtained using powder free of Ni, in which Mo is adhered to the surface of iron-based powder particles by diffusion bonding to achieve a specific surface area of 0.1 m2/g or more.
Further, J P 2015-014048 A (PTL 5) discloses that a sintered body having high strength and high toughness is obtained using powder in which Mo is adhered to iron-based powder particles containing reduced iron powder by diffusion bonding.